Gerber Viewer

• GErbv is a viewer for Gerber RS-274X files, Excellon drill files, and CSV pick-and-place files.
• Gerbv is a native Linux application, and it runs on many common UNIX platforms. A Windows version is also available.
• The core functionality of gerbv is located in a separate library (libgerbv), allowing developers to include Gerber parsing/editing/exporting/ rendering into other programs. Documentation for libgerbv is here.
• Gerbv is one of the utilities affiliated with the gEDA project, an umbrella organization dedicated to producing free software tools for electronic design.

gerbv -- a Gerber (RS-274X) viewer ( https://sourceforge.net/projects/gerbv/files/gerbv/ )

With this website you can visualize your Gerber274x files as well as files in the Excellon 1 and 2 formats and navigate in a fast and simple way through the PCB design.
There is no need to register for using the viewer and also no further installation on your personal computer is required! Just work online with your browser!
Online Gerber viewer ( http://www.gerber-viewer.com/default.aspx )

Available for either MS Windows or Macintosh operating systems

• Easily import 274-X format Gerber plotting documents
• Supports Older 274-D format Gerber plotting documents
• Supports Up to 989 apertures
• Up to 32 plotting film layers*
• Measurement Locator. Absolute and Relative modes
• Direct Aperture Info Query
• Info Query filter options
• Auto-Load Gerber files from desktop
• Supports Printing*
• Supports Conversion to DXF & McCAD document formats*
• Easily re-assign layer coloring
• Easy Layer display control
• Edit Apertures for each Photo Layer
• Create, Save & Load custom Aperture Tables to support older 274D format Documents

Mac CAD. ( http://www.mccad.com/FREE_GerberViewer.html )

ZofzPCB is a FREE 3D Gerber viewer that shows you more than just how your board is going to look. ZofzPCB lets you see inside your board – between the layers – so you can give it a truly thorough inspection. It makes pre-production verification of your PCB designs fast, easy, intuitive and fun.
With ZofzPCB you get:

• Realistic 3D rendering from your Gerber files that shows you just how your finished board Will look and unveils errors that are difficult to detect in a 2D view
• Full 3D navigation via intuitive mouse and keyboard controls, so you can quickly and easily view any point on the board from any angle you choose
• Color scheme selection that lets you assign colors to structures in different ways
• Layer ON/OFF and PEEL functions, which let you remove any design layers that are in your way
• Material density, hollowing and layer spread controls that let you "look between the layers" and verify connections between them
• A measurement function that shows you the X,Y,Z and XY distances between any two points (pad, edge or hole)

ZofzPCB ( https://www.zofzpcb.com )

GerbMagic is a viewer for Gerber RS-274x and Protel ASCII PCB (Prote w2.5 to v2.8 formats). It can also convert Gerber RS-274x and Protel ASCII PCB to PostScript, PDF, TIFF, ...
GerbMagic ( http://softadvice.informer.com/Gerbmagic_3.6.html )